A temperature-dependent positron annihilation 2D-ACAR study of an insulating YBa 2Cu 3O 6.2 single crystal

Abstract

We present and discuss 2D-ACAR positron annihilation spectra in the c-axis projection from an insulating YBa 2Cu 3O 6.2 single crystal at 35 and 300 K, and compare and contrast the results with the corresponding spectra from an untwinned metallic YBa 2Cu 3O 6.9 specimen. The effects of temperature on the 2D-ACAR in the metal and the insulator are similar in size but opposite in sign, i.e. the spectra become more peaked with increasing temperature in the insulator but less so in the metal. The metal/ insulator transition is shown to induce substantial changes in the 2D-ACAR, the spectrum in the insulator being more peaked compared to the metal near p=0. The anisotropic spectrum in the insulator does not contain the characteristic signature of the Cu-O-chain-related ridge Fermi surface, but shows a new cross-like feature with arms extending along the γS directions. We have examined the insulator spectrum regarding possible Fermi surface signatures and found note.